If you signed up for gravatar with the e-mail address you comment with, then you change it there and it’ll change here.

Otherwise your avatar is picked from a pool that Rusche uploads and occasionally changes (this is my third one I’ve had since I started commenting here). It first goes off your e-mail address, and if you leave e-mail blank then it goes off your name. Changing spacing and capitalization changes what avatar you get. However, next time Rusche uploads a new batch of avatars, you’ll be on a new one on both old and new pages.